Two of Pluto’s Moons Get Names From Greek Mythology’s Underworld

An image, taken in 2012, by the Hubble Space Telescope of Pluto and its moons. The two small moons labeled P4 and P5 have been renamed as Kerberos and Styx.Credit
Mark Showalter/SETI Institute; NASA; ESA, via Agence France-Presse — Getty Images

For an astronomical body that circles the Sun but is not officially a planet, Pluto has a lot going on, out there four billion miles from the Sun where comets and other icy bodies roam. Among other things, Pluto has a retinue of at least five — count ’em, five — moons circling it.

On Tuesday the International Astronomical Union announced names for two of these moons, the fourth and fifth to be discovered. Moon No. 4 is now Kerberos, after the many-headed dog that guarded the entrance to the underworld in Greek mythology. Moon No. 5 is Styx, named for the river that souls had to cross over to get to Hades, or the underworld, and the goddess who ruled over it.

Those names, of course, are familiar ones in the Plutonian family. In Greek mythology, Pluto, also known as Hades, was the lord of the underworld, a place that was also sometimes called Hades. Pluto’s other moons are Charon, named for the ferryman who carried dead souls across the river Styx; Hydra, a multiheaded monster that helped guard the entrance to the underworld; and Nix, the mother of Charon.

In 2011 and 2012, Kerberos and Styx were discovered by a team led by Mark Showalter of the SETI Institute, which searches for extraterrestrial life, using the Hubble Space Telescope. Each is estimated to be 20 or so miles in diameter, making them the smallest of Pluto’s moons so far.

By astronomical tradition, the discoverer of a new planet or moon is entitled to suggest names for it to the International Astronomical Union, which has about 11,000 members in 93 countries and was founded in 1919 to promote international scientific cooperation.

Dr. Showalter, however, said he thought it would be more fun to throw it open to the public, and held an Internet contest in which people could vote for names for the moons. He was impressed, he said in an e-mail, by the amount of thinking and creativity that went into the suggestions.

“There were a lot of weird names,” he wrote. “A few suggested ‘Potato and Potahto,’ which I thought was pretty funny. Lots of children wrote in to suggest their siblings as ‘minions of Hades.’ ”

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The favorite name turned out to be Vulcan, which is both the Greek god of fire and, perhaps more significantly, the home planet of Mr. Spock, the “Star Trek” character played by Leonard Nimoy. Dr. Showalter submitted the names Vulcan and Cerberus — which was later changed to the Greek spelling Kerberos to avoid confusion with an asteroid — to the Working Group for Planetary Nomenclature and the Committee on Small Body Nomenclature of International Astronomical Union.

The astronomical union rejected Vulcan because it had already been used as the name for a hypothetical planet between Mercury and the Sun, and it had no connection to the mythological underworld. Instead the moon-namers chose Kerberos and the next runner-up, Styx.

It was not the first time that citizens with stars in their eyes had been disappointed by the astronomical union, which has a tangled history with Pluto. It was the union that, back in 2006, tossed Pluto out of the club of planets, after years of debate that reached into classrooms and planetariums.

In an election at the union’s triennial meeting, in Prague, the group voted to define a planet as an object that was round, orbited the Sun, and had cleared everything else out of its orbital path. Pluto is round and circles the Sun but its region of the solar system is cluttered with thousands of small icy objects.

And so Pluto was demoted to being a “dwarf planet.”

Tell it to Kerberos, Styx, Charon, Nix and Hydra.

Correction: July 5, 2013

An article on Wednesday about the naming of two of Pluto’s moons misidentified the year in which Pluto was demoted from planet status. That decision, by the International Astronomical Union, took place in 2006, not in 2003.

A version of this article appears in print on July 3, 2013, on Page A14 of the New York edition with the headline: Two of Pluto’s Moons Get Names From Greek Mythology’s Underworld. Order Reprints|Today's Paper|Subscribe